It is generally known in the art that the functionality of many electronic devices can be altered by the contact with water, even if only present in traces. In semiconductor devices water can oxidize the electric contacts or chemically alter some parts thereof, or of laser amplifiers used in optical fiber communications. This is described in EP-A-720260.
An electronic application of high industrial interest wherein absence of water is requested are electroluminescent screens based on the use of organic materials, is known in the field as OLEDs (from “Organic Light Emitting Devices”).
The structure of an OLED is formed of a first transparent, essentially planar support, generally made of glass or of a plastic polymer; a first series of transparent linear and mutually parallel electrodes (generally having anode functionality), deposited on the first support; a double layer of different electroluminescent organic materials, of which the first layer is a conductor of electronic vacancies (also defined “holes”) and the second of electrons, deposited on the first set of electrodes; a second series of linear and mutually parallel electrodes (generally having cathode functionality) that are orthogonally oriented with respect to those of the first series, in contact with the upper side of the double layer of organic materials, so that the latter is comprised between both series of electrodes; and a second not necessarily transparent support that may be made of glass, metal or plastics and is substantially planar and parallel to the first support. The two supports are secured to each other along their perimeter, generally by glueing, so that the active part of the structure (electrodes and electroluminescent organic materials) is in a closed space. The first transparent support is the part where the image is visualized, whereas the second support generally has only the function of closing and backing the device, in order to confer mechanical resistance thereto.
The anode is formed of a transparent conductive material, generally a mixed oxide of indium and tin (In2O3—SnO2) which has the features of a semiconductor, known in the field with the acronym ITO (from “Indium Tin Oxide”), whereas the cathode is formed of alkali-earth metals, such as Ba, Ca, and Mg—Ag and Al—Li alloys. When a potential difference is applied to the electrodes, the electrons and the holes are conveyed to the organic material double layer and combine leading to the formation of photons, whose wave length depends on the nature of the organic material used.
For a description of the operating principles of OLEDs and greater details on their structure one can refer to the abundant literature of the field.
A problem encountered with the functioning of OLEDs is their deterioration following to exposure to moisture, which can react with the organic materials (generally polyunsaturated and therefore rather reactive species), as well as with the cathode, formed of particularly reactive metals. The portions concerned with these reactions loose their light-emitting functionality, thus forming black spots on the screen surface.
In order to overcome this problem, international publication WO 99/03122 describes the introduction into the internal space of an OLED of a gas reactive towards water, selected for example among silanes, trimethylaluminum or triethylaluminum. These gases react quickly with the water molecules subtracting them from the internal space of the OLED and generating reaction products which are not detrimental for the functioning of the device. The introduction of a gas in an OLED during the production thereof is however difficult to realize.
U.S. Pat. No. 5,882,761 teaches that the use of solid materials which chemically fix water by remaining in the solid state, such as for example calcium oxide (CaO). A possible problem with the use of this kind of sorbers is that these materials are generally in powder form, and therefore must be retained by a sheet (for example a nonwoven fabric) permeable to water but able to retain the powder particles. Due to the use of the powder material and of the permeable sheet, the minimum thickness of the component intended for water sorption cannot be lower than limit values of about 0.3-0.4 mm, whereas OLEDs manufacturers, in order to fully exploit the potentialities of these flat and thin screens, require moisture sorbing systems which have lower thickness values than the above mentioned ones. Another problem that does not allow the decrease the tickness of the sorbing systems based on the use of CaO or similar is the reduction of the water sorbing capacity.
International publication WO 98/59356 teaches the use of a getter material arranged inside the OLED and fixed onto the second support. This document indicates some alternatives to calcium oxide for water sorption; in particular it indicates the possibility to use materials such as barium, lithium, calcium, barium oxide or similar.
In particular, the metals lithium, barium, and calcium, being particularly reactive towards water, can be used in the devices in limited quantities.
Said metals react with water according to the reaction:2M+2H2O→2M(OH)+H2  (wherein M: lithium)M+2H2O→M(OH)2+H2  (wherein M: barium and calcium)
As it can be noted from the reaction stoichiometry, one or two molecules of metal hydroxides and one molecule of hydrogen are formed every two reacted water molecules. These metals are very reactive, and a drawback is that hydrogen can collect in the OLED thus building up a partial pressure inside the device which can pose safety problems.
Although hydrogen diffuses through the glue used for fixing the two supports of the device and therefore can move outwards, the velocity of hydrogen formation can be higher than the hydrogen permeation velocity through the OLED sealing, thus causing a continuous increase of the gas quantity in the internal space of the device.